Two developing methods have been known in the image forming apparatus using electro-photographic technology—a single-component developing method that uses only toner when developing an electrostatic latent image formed on an image carrying member, and a two-component developing method that uses both toner and carrier. The single-component developing method generally allows the toner to pass through a regulating portion formed between a toner carrying member and a regulating plate pressed to the toner carrying member, whereby toner is charged and a desired thin toner layer is obtained. This method provides the advantages in structure simplification, downsizing and cost cutting. In the meantime, it has the disadvantage of easily accelerating toner deterioration due to the great stress of the regulating portion, and easily reducing the toner charge-receiving property. Further, the regulating member as a charge-applying member for applying charge to toner and the toner carrying member surface are contaminated by toner or external additive agent, whereby the charge-applying property to apply charge to the toner is also reduced. Thus, the amount of charge to be given to toner is reduced, with the result that the service life of the developing device is reduced.
By contrast, in the two-component developing method, the stress is small because the toner is charged by triboelectric charging with toner and carrier mixed, and the carrier has a relatively large surface. Hence the carrier has a great resistance to possible contamination by toner or external additive agent, and this method is advantageous for ensuring a longer service life of the developing device. However, in the two-component developing method, when electrostatic latent image is developed on an image carrying member, a magnetic brush formed with developer slides on the image carrying member surface. This raises an image quality problem wherein traces of the magnetic brush remain on the image. Further, the carrier easily sticks to the image carrying member, thereby causing defects in the image.
One of the conventionally known development methods for solving the image quality problem while maintaining the advantage of longer service life of the two-component developing method is the so-called hybrid development method (Japanese Unexamined Patent Application Publication No. S59-172662), wherein a two-component developer is carried on the developer carrying member and, out of the two-component developer, only toner is supplied to a toner carrying member to perform development. However, the hybrid development method includes the problem wherein the residual toner, on the toner carrying member, not having been used for development appears on the image as hysteresis of development (image memory) in the next development process.
When a half-tone image such as gray is outputted immediately after the image of high contrast such as a solid black image on a white background, the previously printed pattern of high contrast appears in the half-tone image. This phenomenon is the aforementioned image memory and due to the unevenness of an image density caused by the unevenness of the toner layer created as follows. A toner layer not having been used for development remains on the toner carrying member corresponding to the printed image pattern after printing of the high contrast image, and unevenness of thickness of the toner layer corresponding to the printed image pattern is created on the toner carrying member after the next toner supply process.
To solve the problem of the image memory in the hybrid development method, a proposal has been made to employ a developer carrying member for collecting the toner, not having been used for development, from a toner carrying member, in addition to a developer carrying member for toner supply which supplies toner to the toner carrying member (Japanese Unexamined Patent Application Publication No. H10-319708). This method ensures that the toner not having been used for development remaining on the toner carrying member is collected onto the developer carrying member for collecting toner, whereby generation of an image memory can be prevented. However, the technique disclosed in the Japanese Unexamined Patent Application Publication No. 10-319708 has a problem that the carrier is deteriorated due to a long-term use and the amount of charge to be charged to toner is reduced, whereby image quality is deteriorated.
The present inventors have made concentrated study efforts to solve the aforementioned problems, and have proposed a technique of achieving a still longer service life in the two-component developing method, wherein the reverse polarity particles having a polarity reverse to that of the charged toner are added to the developer containing both toner and carrier, thereby compensating for the shortage of the charging property of the carrier (Japanese Unexamined Patent Application Publication No. 2007-108673). This technique is also applicable to the hybrid development method, and further prolongs the service life of the hybrid development method. However, the technique disclosed in the Japanese Unexamined Patent Application Publication No. 2007-108673 still has the problem of generating an image memory.
In view of the prior art problems described above, it is an object of the present invention to provide a developing device and an image forming apparatus capable of forming a high-quality image for a long period of time while preventing the occurrence of image memory. Another object of the present invention is to provide a developing device and an image forming apparatus capable of preventing both the occurrence of image memory and reduction in the amount of charge to be charged.